PLANT PHYSIOLOGY , Vol 115, Issue 4 1671-1680, Copyright © 1997 by American Society of Plant Biologists
|
WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Effect of Nitrogen Nutrition on Remobilization of Protein Sulfur in the Leaves of Vegetative Soybean and Associated Changes in Soluble Sulfur Metabolites
Sunarpi and J. W. Anderson
School of Botany, La Trobe University, Bundoora, Victoria 3083, Australia
The hypothesis that protein S is remobilized from mature leaves in response
to N stress but not S stress was examined by transferring vegetative
soybean (Glycine max L. Merr) plants grown with adequate sulfate and
nitrate to nutrient medium with low sulfate (5 [mu]M) and nitrate at either
15, 7.5, 2, or 0.25 mM. Soluble S decreased to very low levels in mature
and maturing leaves, especially in low-N plants. At high [N], insoluble S
(protein) in mature leaves remained constant, but at low [N], after the
soluble S declined, up to 40% of the insoluble S was exported. The losses
were complemented by gains, initially in soluble S, but subsequently in
insoluble S, in the expanding leaves and the root. In low-N plants, but not
in high-N plants, the decrease in insoluble S in mature leaves was
complemented by increases in homoglutathione (hGSH), Cys, and Met. At low
[N], but not at high [N], the developing leaf, leaf 5, contained high
amounts of soluble S, mostly hGSH. The results suggest that, at low [N],
protein S is metabolized to hGSH, which serves as the principal transport
compound for the export of organic S.
